The present invention relates to mobile sound field correcting devices, and more particularly to a mobile sound field correcting device for correcting acoustic characteristics in a sound field in a vehicle formed by a directed wave radiated by a loudspeaker and a reflected wave.
The acoustic wave patterns in a vehicle are significantly different than in a building or home because the sound field is surrounded by reflective surfaces such as glass and metal plates and the sound chamber inside the vehicle is generally small. Further, because the front seat is located substantially at the middle of the vehicle and the rear seat to the rear of the vehicle, the front seat and the rear seat have different acoustic characteristics. Accordingly, the reflected waves arriving at the ears of persons seated in the front and rear seats have different paths and different characteristics. Furthermore, the direct and reflected waves have different travel distances, and therefore the front and rear seats are greatly different in acoustic characteristics such as the sound pressure versus frequency characteristic and sound wave phase characteristic.
Examples of the sound pressure versus frequency characteristic are shown in FIGS. 1A and 1B. FIG. 1A indicates the sound pressure versus frequency characteristic at the front seat in the case where front sound sources (loudspeakers) are installed on the right and left sides of the front doors, and FIG. 1B indicates the sound pressure versus frequency characteristic at the rear seat in the case where rear sound sources are installed on the parcel tray.
Accordingly, even if loudspeakers which exhibit excellent characteristics in an anechoic room are installed in a vehicle, it is impossible to obtain a flat frequency characteristic in the vehicle. Therefore, a frequency characteristic varying device is inserted in the reproduction system of the mobile acoustic device to correct for the irregularities in the frequency characteristic.
An example of the reproduction system of a mobile acoustic device is shown in FIG. 2. In FIG. 2, an audio signal from a signal source 1 such as a tuner or a cassette deck is amplified by an amplifier 2 and is then applied to a frequency characteristic amplifier 3. The output of the amplifier 3, after being subjected to power amplification by a loudspeaker drive signal to a loudspeaker 5.
In the above-described circuit, the audio signal from the signal source is converted into a signal having a normal frequency characteristic correcting amplifier 3, which may be a graphic equalizer or bass/treble tone control, to have a desired frequency characteristic, as determined by the characteristics of the space in which the system is being employed, and then applied as the loudspeaker drive signal.
With the mobile acoustic system, sound waves radiated from a loudspeaker 5 reach a listening point 6 in a sound field as shown in FIG. 3. More specifically, a direct wave from the loudspeaker advances along a path 7 (indicated by a solid line 8) to the listening point 6, and a reflected wave reflected by a wall or the like in the vehicle advances along a path 8 (indicated by dotted lines 8) to reach the listening point 6. In this case, the direct wave and the reflected wave interfere with each other at the listening point 6. In this case, the direct wave and the reflected wave interfere with each other at the listening point 6 so that peaks and dips occur at certain frequencies, and hence the frequency characteristic at the listening point 6 is made irregular.
The reason why peaks and dips occur in the frequency characteristic will be briefly described.
If it is assumed that, in FIG. 3, the length of the direct wave path 7 to the listening point 6 is represented by I.sub.1, the length of the reflected wave path 8 to the listening point 6 is represented by I.sub.2, and the sound velocity is represented by c, then a sound wave S.sub.M at the listening point 6 is the sum of the direct wave S.sub.S and the reflection wave S.sub.R. ##EQU1## where K is the reflection factor of an internal wall or the like and A is the signal strength in the vibration plane of the loudspeaker.
As for the reflected wave S.sub.R, in almost all cases it can be considered that the reflections are of the fixed-edge type, and therefore it can be considered that the acoustic impedance of wall is higher than that of an air and that reflections occur in the same phase. (In the case of a free-edge type reflection, the acoustic impedance is small, and S.sub.M =S.sub.S -S.sub.R.)
Equation (1) can be rewritten as follows: EQU S.sub.M =(A/I.sub.1)(e.sup.j.omega.(t-I.sbsp.1.sup./c) +Be.sup.j.omega.(t-I.sbsp.2.sup./c)), (2)
where B=K I.sub.1 /I.sub.2.
Assuming that B=1 for simplification in description, the frequencies at which peaks occur is: EQU f=nc/(I.sub.1 -I.sub.2), (3)
and the frequencies at which dips (troughs) occur is: EQU f=(n+1/2)c/(I.sub.2 -I.sub.2), (4)
where n is an integer.
These equations are graphed in FIG. 4. In practice, B.noteq.1, and therefore the sound pressure at the dip frequencies cannot be zero. Also, at the peak frequencies, S.sub.M .noteq.2A/I.sub.1. In FIG. 4, the dotted line indicates the results in the case of free-edge type reflections for the purpose of reference.
As is apparent from the above description, the irregularities in the frequency characteristic due to the interference of the reflected wave are attributed to the difference in travel distance between the two sound waves. Therefore, if the physical arrangement of the device or the listening position is changed, the peak and dip frequencies are changed. To eliminate this difficulty, a number of graphic equalizers can be used, but this increases the system cost. Moreover, it is considerably difficult to adjust such graphic equalizers. Furthermore, in the case of a device which has a bass/treble control function only, it is impossible to fully correct for peaks and dips.
Inside the vehicle, the high-frequency response at the listening point is lowered, in an amount depending on the sound absorption characteristics of interior materials and the installation positions of the loudspeakers, because the attenuation factor and the sound absorption factor generally increase with frequency. The high-frequency characteristic at the front seat is more strong affected than that at the rear seat. This is due to the fact that the rear window is reflective.
Furthermore, inside the vehicle the perceived frequency characteristic is varied by the amount of background noise inside the vehicle. FIG. 5 shows typical spectra of noise inside a vehicle with road conditions and travel speeds varied.
As is apparent from the above description, the acoustic characteristic inside a vehicle varies according to the listening position, the materials of which components inside the vehicle are constructed, and the ambient noise level.